Steam turbine components made of Cr-Mo-V alloys, such as rotors and discs, provide optimum high-temperature fatigue and creep properties, but are considered difficult to weld. However, since the down time associated with replacement of these often worn, eroded, or cracked components can cost electric utilities hundreds of thousands of dollars per day, many procedures have been attempted to repair them.
One such repair procedure consists of welding an individual piece of forged steel to a worn rotor or disc. However, when this type of repair is made on a single rotor blade groove fastening, herein referred to as a "steeple", welder accessibility is very limited. Accordingly, a weld repair conducted with very limited accessibility can result in unacceptable, non-destructive examination quality due to the formation of porosity cracks and slag inclusions.
It is also known to make rotor repairs by submerged arc welding after a low volume welded seam is made between a turbine component and a forged replacement section. See Kuhnen, U.S. Pat. Nos. 4,213,025 and 4,219,717, which are herein incorporated by reference. In such a procedure, a ring forging is welded to a worn disc or rotor or a completely new rotor forging is welded to replace the entire end of the rotor. See Clark et al. U.S. Pat. No. 4,633,554, disclosing a narrow gap weld root pass followed by a gas metal arc build-up for this purpose.
The lower tensile and fatigue properties obtained by employing this process, however, are generally insufficient for use in high stress rotor steeple areas.
Submerged arc welding alone has also been used for build-up repairs of rotor areas involving a wide or deep groove, where a cracked defect is not oriented longitudinally along the radius of the rotor. The main advantage of building up with submerged arc welding is that this procedure has a very high deposition rate, typically about 15 pounds of weld metal per hour. The higher deposition rate is important since many of the service rotor weld repairs are made during turbine outages, thus, time is extremely important. However, this process requires a pre-heat, produces a relatively large grain size with inferior metallurgical properties. Typically, these submerged arc weldments on low pressure rotors have a yield strength of about 85 to 100 ksi (586 to 689 MPa) and a room temperature Charpy toughness of about 100 to 120 ft-lbs (136 to 163 J). It is also understood that submerged arc weldments are often rejected due to poor ultrasonic quality, which often reveals slag inclusions and porosity in the weld metal. Moreover, serious creep-rupture and notch-sensitivity problems have been encountered for high pressure Cr-MO-V rotor welds manufactured from submerged arc weldments. Thus, the submerged arc process is generally unacceptable for use for weld repairs of Cr-Mo-V rotor steeples having small, high-stress concentration radii.
Gas metal arc procedures have also been employed for repairing rotors and discs. This welding procedure deposits about 8 lbs of weld metal per hour, typically having slightly better properties than weldments produced by the submerged arc process. Gas metal arc weldments of alloy steel turbine components generally have a yield strength of about 85 to 100 ksi (586 to 689 MPa), and a room temperature Charpy toughness of about 110 to 130 ft-lbs (150 to 177 J). For Cr-Mo-V rotor repair welding, the gas metal arc welding process, however, is often associated with arc-blow (magnetic) process limitations when used with Cr-Mo-V alloys.
Recently, emphasis has been placed on the use of gas tungsten arc welding processes (GTAW) for making repairs on Ni-Mo-V and Ni-Cr-Mo-V low pressure rotor components. See R. E. Clark, et al. "Experiences with Weld Repair of Low Pressure Steam Turbine Rotors", 47th American Power Conference, Apr. 22-24, 1985, Chicago, Ill., printed by Westinghouse Electric Corporation, Power Generation, Orlando, Fla., herein incorporated by reference. Gas tungsten arc welding has been employed for repairing individual rotor attachment grooves, cosmetic, or shallow groove repairs to correct minor surface defects. It has also been used to allow multiple build-ups of plate attachment groove locations, i.e., for a 360.degree. application, and cladding to restore worn-away material. Gas tungsten arc welding offers relatively high ultrasonic quality, requires no pre-heat, and produces weldments having tensile and impact properties which exceed rotor material specification requirements. Low alloy steel weldments produced by this process nominally have a yield strength of about 90 to 115 ksi (621 to 793 MPa), and a room temperature Charpy toughness of about 160 to 210 ft-lbs (218 to 286 J). In addition, this welding procedure produces the finest microstructural grain size of any of the above-mentioned processes.
The selection of a weld method depends on factors such as distortion, non-destructive testing acceptance limits, and mechanical property response to the post-weld heat treatment. Each area of a turbine rotor is unique, and experiences a different service duty. The absence of weld and heat affected zone cracking as well as the minimization of defects, can only be accomplished by carefully controlling a number of welding variables. For the gas tungsten arc welding process, some of these variables include amperage, alloy selection, joint geometries and travel rate. The parameters selected should be accommodating to automatic welding processes to obtain a uniform quality which is reproducible from weld to weld. These parameters must also produce superior welding characteristics such as freedom from porosity, CraCking, and slag entrapment, while being accommodating to all possible repairs on rotors and discs. Finally, the alloy and welding parameters selected must produce a weld comparable to the properties of the base metal.
Accordingly, a need exists for a welding procedure that maximizes the metallurgical properties of the repaired area of turbine components. There is also a need for a welding procedure that minimizes the heat affected zone and eliminates weld-related cracking.